Listeria monocytogenes (Lm) is an intracellular pathogen that primarily infects antigen presenting cells and has adapted for life in the cytoplasm of these cells. Listeria monocytogenes and a protein it produces named listeriolysin O (LLO) have strong adjuvant properties that unlike the majority of adjuvants used for cellular based immunotherapies, can be administered after providing an antigen specific treatment.
Tregs play a critical role in the maintenance of peripheral self-tolerance. Naturally occurring CD4+CD25hi Tregs are produced in the thymus and express FoxP3, a transcriptional factor required for establishment and maintenance of Treg lineage identity and suppressor function. Tregs can accumulate at a disease site, where they suppress the effector function of disease specific T cells. When this occurs it can result in an increase in disease despite the presence of appropriate antigens or T cells activated to attack those antigens. Increased densities of tumor-infiltrating FoxP3+ Tregs have been associated with poor prognosis in various solid tumors, including pancreatic, ovarian, and hepatocellular carcinoma. Depletion of Tregs results in enhanced antitumor immunity and tumor rejection in murine models but may also result in the development of autoimmune diseases.
Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of early myeloid progenitors, immature granulocytes, macrophages, and dendritic cells at different stages of differentiation. These cells are of great interest because they have the capacity to suppress both the cytotoxic activities of natural killer (NK) and NKT cells, and the adaptive immune response mediated by CD8+ T cells. While the mechanism of NK cell inhibition is currently not well-understood, multiple pathways are responsible for MDSC-mediated T cell suppression including: 1) production of arginase 1/ARG1 and 2) upregulation of nitric oxide synthase 2 (NOS2). ARG1 and NOS2 metabolize L-arginine and either together, or separately, block translation of the T cell CD3 zeta chain, inhibit T cell proliferation, and promote T cell apoptosis. Additionally, MDSCs secrete immunosuppressive cytokines and induce regulatory T cell development. In mice, MDSCs are broadly defined as CD11b+Gr-1/Ly-6G+ cells, but the relative expression levels of Ly-6G and Ly-6C identify two specific subsets. Human MDSCs commonly express Siglec-3/CD33 and lack lineage markers and HLA-DR, but heterogeneous expression of CD14 and CD15 suggest that multiple subsets exist.
MDSCs are induced by pro-inflammatory cytokines and are found in increased numbers in infectious and inflammatory pathological conditions. They accumulate in the blood, bone marrow, and secondary lymphoid organs of tumor-bearing mice and their presence in the tumor microenvironment has been suggested to have a causative role in promoting tumor-associated immune suppression. Although it is now evident that MDSCs may serve as a target for preventing tumor progression, further characterization is necessary to determine effective mechanisms by which they can be inhibited.
The invention provides an effective mechanism of inhibiting suppressor cells such as Tregs and MDSCs by providing Listeria vaccines that once administered to tumor-bearing subject, proceed to suppress Tregs and MDSC function, thereby allowing anti-tumor T cells to replicate and inhibit tumor growth.